Horizontal floor and roof diaphragms transfer lateral load to walls and other supporting elements. The amount each wall assumes depends if diaphragms are flexible or rigid.

1. Flexible diaphragm

Floors and roofs with plywood sheathing are usually flexible; they transfer load, similar to simple beams, in proportion to the tributary area of each wall. Wall reactions R are computed based on tributary area of each wall. Required shear flow q (wall capacity)

2. Rigid diaphragm

Concrete slabs and some steel decks are rigid; they transfer load in proportion to the relative stiffness of each wall. Since rigid diaphragms experience only minor deflections under load they impose equal drift on walls of equal length and stiffness. For unequal walls reactions are proportional to a resistance factor r.